There are currently no acceptable methods for decontaminating sensitive equipment such as electronics, optics, and artworks which have been exposed to chemical or biological warfare (CBW) agents including anthrax, mustard blistering agent, VX nerve gas, and the like. Current candidate technologies include: (1) solvent washing; (2) low-pressure plasmas; (3) supercritical carbon dioxide (SCCO.sub.2); (4) reactive foams and gels; and (5) atmospheric-pressure plasmas. Solvent washing uses chlorinated fluorocarbon replacement solvents to remove CBW agents, thereby contaminating the solvent and necessitating frequent replacement and decontamination or acceptable disposal of the solvent. It is also unclear how effective this method is, particularly against biological warfare agents. Low-pressure plasmas have potential but, typically, are not very penetrating and are limited to materials that can survive being subjected to a vacuum. At sub-torr pressures, reactive species must rely on diffusion to penetrate into cavities and crevices which are often beyond the spread of the plasma, thereby restricting this procedure's usefulness for other than smooth, vacuum-compatible objects having only external contamination. By employing pressure pulsing to pressures above about 100 torr in a decontamination chamber, convection will augment the transport of reactive species, thereby enhancing penetration into cavities and crevices. Supercritical CO.sub.2 has shown promise for removal of chemical warfare agents; however, this process requires secondary separation and neutralization of the agent. Moreover, the high pressure of the super-critcal point (.about.73 atm) may restrict the types of materials that can be decontaminated. Hermetically sealed equipment and certain polymers, as examples, are at risk. Reactive foams and gels may be of use, but aqueous content and lingering residues may degrade performance of sensitive equipment.
The standard for sterilization involves steam treatment at 121.degree. C. and 15 psi above atmospheric pressure in an autoclave. This procedure can be used only for articles which can withstand moisture and heat under pressure, and excludes materials and equipment such as endoscopes and surgical sharps. Dry heat at 165.degree. C. may be used for moisture-sensitive but not heat-sensitive materials. Ethylene oxide (EtO) is the industry standard for low-temperature sterilization, but also raises many difficulties. Hospitals have been reducing their dependence on EtO due to its extreme toxicity, flammability and environmental consequences. Furthermore, a sterilization cycle using EtO takes approximately 18 hours to complete and is expensive and inconvenient, since highly trained operators are required. Ionizing radiation has been accepted for certain applications; however, public concern over safety and the environment continue to be issues which must be overcome. Liquid disinfectants, such as peracetic acid, cannot be used on moisture-sensitive materials and are hazardous, which leads to environmental concerns regarding their disposal. Low-pressure hydrogen peroxide plasma sterilization has recently been introduced. It is thought that hydrogen peroxide vapor is solely responsible for the sterilization, while the plasma merely decomposes the hydrogen peroxide after the sterilization process so that residuals are not left on surfaces. Again, this process cannot be used for moisture-sensitive materials and, as stated hereinabove, low-pressure plasmas are not attractive for sterilization due to the poor penetration capability of the short-lived reactive species by diffusion processes, the requirement that the materials be vacuum-compatible, and the cost of vacuum generating equipment.
Atmospheric-pressure plasmas are useful for both removal of unwanted materials from substrates and neutralization/ sterilization thereof without damaging most substrates. As a sterilization method for the health care and food industries, atmospheric pressure plasmas offer many advantages over existing methods. Atmospheric pressure plasmas can be non-thermal (cold) plasmas, or thermal (hot) plasmas. Traditional cold atmospheric-pressure plasmas, such as the corona discharge and the dielectric-barrier or silent-discharge plasma, are highly non-uniform and are typically used for volume processing of gaseous effluents or as ozone generators. Emerging cold atmospheric-pressure technologies include a one atmosphere uniform glow discharge plasma described in "Room Temperature Sterilization of Surfaces And Fabrics With A One Atmosphere Uniform Glow Discharge Plasma" by K. Kelly-Wintenberg et al., J. Indust. Microbio. & Biotech. 20, 69 (1998). This device generates a uniform plasma and, in the case of oxygen containing plasmas, favors the preferable production of atomic oxygen over ozone. However, only low-power densities can be achieved.
The atmospheric-pressure plasma jet (APPJ) is a non-thermal, high-pressure uniform-glow plasma discharge that produces a high-velocity effluent stream of reactive chemical species. See, e.g., "The Atmospheric-Pressure Plasma Jet: A Review And Comparison To Other Plasma Sources" by A. Schutze et al., IEEE Trans. Plasma Sci. 26, 1685 (1998). The discharge operates on a feedstock gas such as He/O.sub.2 /H.sub.2 O, which flows between an outer, grounded, cylindrical electrode and an inner, coaxial electrode powered at 13.56 MHz. While passing through the plasma, the feedstock gas becomes excited, dissociated or ionized by electron impact. Once the gas exits the discharge volume, ions and electrons are rapidly lost by recombination, but the fast-flowing effluent still contains neutral metastable species (for example, O.sub.2 * and He*) and radicals (for example, O and OH).
The use of the atmospheric-pressure plasma jet for decontamination of chemical and biological warfare agents is described in "Decontamination Of Chemical And Biological Warfare (CBW) Agents Using An Atmospheric Pressure Plasma Jet (APPJ)" by H. W. Herrmann et al., Phys. Plasmas 6, 2284 (1999). The reactive effluent from an APPJ has been shown to be an effective neutralizer of surrogates for anthrax spores and mustard blistering agent. Unlike conventional decontamination methods, the plasma effluent was observed not to cause corrosion or destroy wiring, electronics, or most plastics, rendering it suitable for decontamination of sensitive equipment and interior spaces. Furthermore, the reactive species in the effluent were observed to degrade into harmless products leaving no residues or harmful by-products. The APPJ can be run at high power densities, unlike other cold discharges, which results in higher fluxes of reactive species.
Hot atmospheric-pressure plasmas, such as dc arc jets and rf plasma torches, operate at several thousand degrees Celsius which is too hot for most decontamination applications. Although the APPJ operates at somewhat higher temperatures than other cold discharges, APPJ exposure temperatures can be maintained in an acceptable range for most decontamination applications (that is, between 50.degree. C. and 300.degree. C.). Moreover, these slightly elevated temperatures often produce a desirable effect by increasing reaction rates.
Unlike the other cold atmospheric-pressure plasmas, the APPJ requires helium in the feedstock gas. The feedstock is vented to areas surrounding the APPJ, thereby permitting the helium to irretrievably escape as well as allowing the escape of re-aerosolized agents or harmful byproducts thereof. Although operation of an APPJ using an alternative feed gas, such as air, may be possible, there have been no reports of such operation.
Accordingly, it is an object of the present invention to provide an atmospheric-pressure plasma sterilization chamber capable of minimizing loss of helium.
Another object of the present invention is to provide an atmospheric-pressure plasma sterilization chamber capable of recirculating the helium from the feedstock gas and preventing the escape of re-aerosolized agents or harmful byproducts thereof.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.